1. Field of the Invention
The present invention relates to an apodised binary grating. In particular, although not exclusively, the invention relates to the apodisation of distributed Bragg gratings for use in optical fibres and devices.
2. Description of the Related Art
It will be understood that the terms “optical” and “optoelectronic” are used in this specification in a non-specific sense, that is so as to cover use with radiation in the visible and non-visible parts of the spectrum, and so as not to be limited to use with visible light. Similarly, it will be understood that use of the term “light” may apply to electromagnetic radiation of any frequency, and is not limited to light in the visible spectrum. Further it will be understood that the use of the term “waveguide” describes a structure that guides light and which may comprise a plurality of layers.
Distributed Bragg gratings are commonly found in optical fibres and semiconductor optical devices. Such gratings in optical fibres are known as “Fibre Bragg Gratings” (FBGs) and those in semiconductor optical devices as “Distributed Bragg Reflectors” (DBRs).
In its simplest form, a Bragg grating comprises a periodic modulation of the refractive index of a waveguide. Light is scattered at each change in refractive index. If the Bragg condition is satisfied, the light reflected at each of the grating planes interferes constructively. The Bragg condition is defined as λB=2neffΛ, where λB is the wavelength of the incident light, neff is the effective refractive index of the waveguide, and Λ is the pitch of the modulation. A grating of constant pitch and reflective strength thus produces a reflection of light of a wavelength of twice the effective pitch of the grating, where the effective pitch differs from actual pitch by a factor of neff.
However, as well as a reflective peak at the principle wavelength (λB) the grating also produces other unwanted reflections (typically smaller than the principle peak) at side wavelengths, due to the abrupt termination of the grating caused by its finite length. Bragg gratings can be adapted to reflect a range of wavelengths, and these are known as chirped gratings. The pitch of a chirped grating varies along the length of the grating, typically monotonically. Chirped gratings are often used in tunable semiconductor lasers, and an example is shown in WO03/012936. Further examples may be seen in U.S. Pat. No. 6,771,687, which provides an example of how FBGs may be used in an FBG stabilised laser, and U.S. Pat. No. 6,345,135, which illustrates applications of DBRs in semiconductor optoelectronic devices. A chirped grating of constant reflective strength can produce a reflection spectrum (reflectivity plotted against wavelength) in the shape of a “top hat”, i.e. the reflection of the grating is substantially uniform within a specific wavelength range. However, the finite length of the grating again causes unwanted peaks in the reflection spectrum, the two largest of which will typically coincide in wavelength with the ends of the top hat profile, producing raised peaks at the ends of the profile with higher reflectivity than the middle section.
It is known in the art that controlling the reflectivity of the grating, in particular close to the ends of the grating, can be used to overcome the problems of unwanted peaks caused by the finite length and abrupt termination of the grating, in both constant pitch and chirped gratings. This is known as apodisation. Apodisation can also be applied to other similar ‘end effects’ in other types of known grating. In the context of distributed Bragg gratings, the term apodisation is generally used to describe varying the strength of the grating (i.e. the reflectivity) as a function of grating length.
Known techniques for achieving apodisation of Bragg gratings are described in “Fiber Bragg Gratings” by Raman Kashyap [ISBN 0124005608] and “Fiber Bragg Gratings” by Andreas Othonos [ISBN 0890063443]. The variation in reflectivity is generally achieved by controlling the contrast of an optical exposure pattern, a technique that might be described as ‘intensity modulation apodisation’. FIG. 1 shows how the local effective refractive index n varies along the waveguide at the end of a known apodised Bragg grating. The figure shows three regions. In region 1, which represents the end of the main part of the grating, the variation in refractive index (and thus the reflectivity at each period of the grating) is constant. In region 2, the variation in refractive index is gradually reduced, reducing the reflectivity of each period of the grating. In region 3 the refractive index is constant and this represents the region beyond the end of the grating.
Bragg gratings such as FBGs and DBRs are most commonly defined by holographic techniques, i.e. by means of an optical interference pattern. FBGs are typically ‘written’ with high intensity UV radiation, which is used to create patterned refractive index ‘change’ along the length of an optical fibre. In the case of DBRs the holographic pattern is used to create a lithographic exposure pattern on a photosensitive chemical resist (photoresist), which can then be developed and used as a lithographic etch mask to transfer a “binary” approximation of the interference pattern onto the semiconductor material (the pattern becomes binary due to the threshold of exposure of the photoresist, thus providing regions with and regions without photoresist).
Known FBGs are typically apodised by intensity modulating the holographic exposure. Typically the intensity variation is reduced at the ends of a grating to provide a weakening of the reflective strength of the grating in these regions and this can prevent or reduce the unwanted side peaks in the optical reflection spectrum of light transmitted along the fibre.
The approach of intensity modulation apodisation can also be applied to DBRs: a variable intensity exposure can be generated for exposing the photoresist. Adequate exposure of the photoresist requires a threshold ‘dose’ of light. An apodised holographic exposure pattern will lead to the formation of a photoresist etch mask with a corresponding variation in its mark:space ratio, which is then transferred to the semiconductor during the subsequent etch processing step. This is illustrated in FIGS. 2A and 2B, which show the end portions of a non-apodised grating 4 and a grating 5 apodised by changing exposure along its length. In the non-apodised grating 4 shown in FIG. 2A, the mark:space ratio between the widths of the marks 6 and spaces 7 remains constant as far as the end 8 of the grating. In the apodised grating 5 shown in FIG. 2B, the mark:space ratio decreases from 1:1 towards the end 9 of the grating, with a corresponding decay in reflection strength.
For first order gratings the reflective strength of the grating is greatest when the mark:space ratio is 1:1, and is weaker either side of this, which requires careful control of the exposure dose used in the lithographic pattern during manufacture. The objective of reducing the reflective strength in a region at the end of a DBR grating is typically achieved by either reducing or increasing the mark:space ratio in that region, by means of varying the exposure.
The exposure of photoresist and transfer of the exposure pattern to the semiconductor by etching can be difficult to control to a high level of precision. Consequently it is not uncommon for there to be a processing tolerance range for the mark:space ratio of the transferred pattern, which is, for example centred on the maximal grating strength (for a first order grating) of 1:1 mark:space ratio.
However, since the apodisation pattern relies upon the contrast of the exposure pattern being weakened at the ends of the grating, in order to produce a comparable change in the mark:space ratio, this can result in an undesirable feature for some of the gratings within the processing tolerance range: for some gratings, where the mark:space ratio is not exactly 1:1, the effect of apodisation may be that the grating strength initially increases as the ratio is changed before it decreases. This is illustrated in FIG. 2C, which shows the end portion 10 of a non-apodised grating with a mark:space ratio greater than 1:1, and apodised grating sections 11, 12 with successively reduced mark:space ratios. The first apodised section 11 has a mark:space ratio close to 1, leading to a greater reflective grating strength than the non-apodised section 10. This produces a grating with undesirable optical properties, and has a detrimental effect on manufacturing yield. This difficulty with controlling the mark:space ratio of the intensity modulation apodisation also applies to FBGs made by non-holographic techniques.
An alternative means for writing DBRs is by means of electron beam lithography (also known as “e-beam”). In this technique a lithographic resist is used that is sensitive to e-beam exposure (i.e. “e-beam resist”), and the desired lithographic pattern is written directly on the resist with the e-beam. Due to the directly programmable nature of the e-beam writer, this technique offers greater flexibility than the holographic technique in many respects, making it suitable for producing complex gratings. However, e-beam writers have limitations when producing intensity modulation apodisation.
A further alternative means for writing DBRs is by means of a photolithographic exposure mask. In this technique light, typically UV-light, is incident on a photoresist through an opaque photolithographic mask on an optically transparent plate. This technique is particularly suitable for higher order gratings. However, this technique is typically used with a constant level of illumination across the mask, and so is also not suitable for intensity modulation apodisation.
Thus there is a need in the art for an apodised grating that does not rely upon intensity modulation apodisation.